In an image forming apparatus such as a laser printer, a developing cartridge is detachably mounted in an apparatus main body. Toner is accommodated within the developing cartridge. When toner in the developing cartridge is used up, the developing cartridge is removed from the apparatus main body. Then, a new developing cartridge is mounted in the apparatus main body. In addition, when a sheet is jammed in the apparatus main body, there may be a situation in which the developing cartridge is removed from the apparatus main body, and after the jam is resolved, the developing cartridge is remounted in the apparatus main body.
In this type of image forming apparatuses, there is proposed an image forming apparatus in which a detection gear having an abutment projection is provided on a side surface of a developing cartridge, and when the developing cartridge is mounted in an apparatus main body, information on the developing cartridge is obtained based on rotation of the detection gear.
The detection gear is provided to be rotatable about an axis which extends in a direction which orthogonally intersects the side surface of the developing cartridge. Gear teeth are formed on a circumferential surface of the detection gear except a part thereof. Namely, the detection gear is a partly non-tooth gear. In addition, a transmission gear is provided on the side surface of the developing cartridge to be rotatable about an axis which extends in parallel to the axis of the detection gear with a space therebetween. Gear teeth are formed on a circumferential surface of the transmission gear so as to extend along the full circumference thereof. With a new developing cartridge, the gear teeth of the detection gear mesh with the gear teeth of the detection gear. When the developing cartridge is mounted in the apparatus main body, a driving force of a motor is inputted into the transmission gear, and the driving force is transmitted from the detection gear to the detection gear via the gear teeth of these gears.
With the driving force so transmitted, the detection gear rotates, and the abutment projection moves as the detection gear rotates. A sensor is provided in the apparatus main body for detecting a passage of the abutment projection. Then, whether the developing cartridge is new or used is determined based on whether or not the passage of the abutment projection is detected by the sensor within a predetermined length of time after the start of driving of the motor. When the detection gear continues to rotate so that a non-tooth portion of the detection gear comes to oppose the gear teeth of the transmission gear, the mesh engagement of the gear teeth of the transmission gear with the gear teeth of the detection gear is released, whereby the detection gear stops rotating (for example, see JP-A-2006-267994).